1. Field of the Invention
The present invention relates to a vehicle, especially a working vehicle like a tractor, improved in its steering. Firstly, the improvement is provided for the vehicle installed with an integral transaxle apparatus comprising a hydrostatic and mechanical axle driving and steering system including two hydrostatic transmissions (hereinafter, each hydrostatic transmission is called an “HST”); one for driving left and right axles, and the other for steering the axles, wherein the input of the HST for steering is independent of the output of the HST for driving. Secondly, the improvement is for the vehicle having a caster in addition to its driving wheels.
2. Related Art
A well-known conventional system for driving and steering left and right axles for wheels or sprockets of tracks, which employs a pair of HSTs provided for the respective axles, is disclosed in U.S. Pat. No. 4,782,650 or Japanese Laid Open Gazette No. Hei 2-261,952, for example. Output rotary speeds of the two HSTs are equalized for straight driving of a vehicle and made different from each other for steering of it.
The conventional system has the problem that the equalization of output rotary speeds between the two HSTs must be precise and if there is difference of capacity between the two HSTs, a vehicle was steered differently whether leftward or rightward.
The present axle driving and steering system includes two HSTs and two differentials, wherein one (a driving HST) of the HSTs interlocks with a speed changing operation means like a lever or a pedal and transmits power from a prime mover like an engine into one differential connecting left and right axles for driving them. The other HST (a steering HST) interlocks with a steering operation means like a steering wheel and transmits its output rotational force corresponding to the direction and degree of the steering operation means into the other differential, and a pair of differential output shafts of the latter differential are drivingly connected to the respective axles through two drive trains which are opposite in their output rotational directions.
Furthermore, two types of such an axle driving and steering system have been provided. One is a dependent steering type, wherein the steering HST receives the driving power for its hydraulic pump from the output of the hydraulic motor of the driving HST. The other is an independent steering type, wherein the steering HST receives the driving power for its hydraulic pump from the prime mover independently of the output of the driving HST.
In comparison with the dependent steering type system, the independent type system has some advantages as follows:
Firstly, the steering HST can be driven while the speed changing operating means is in neutral so as not to drive the driving HST, so that both the left and right axles are rotated oppositely to each other at an equal speed, whereby a vehicle installed with the system can swivel in place.
Secondly, the volume of the driving HST can be reduced because it does not have to drive the steering HST.
Thirdly, while the steering HST of the dependent steering type must have so large volume as to compensate the low efficiency of rotation of its input shaft (a pump shaft), the volume of the independent steering type one, which has a pump shaft independently driven by a prime mover, does not have to be so large. However, a vehicle installed with the independent steering type system has the following disadvantage:
If a vehicle employing the independent steering type system is instituted so that the lateral direction of the steered vehicle coincides with that of an operated steering operating means (like a steering wheel) during forward travelling, the vehicle comes to be steered oppositely to the operational direction of the steering operating means during backward travelling. For example, when a steering wheel is rotated leftward during backward travelling, the vehicle turns rightward. Such a situation confuses an operator accustomed to driving a normal car.
Also, there has never been disclosed a compact integral construction employing the above mentioned system comprising two axles, two HSTs; one for driving a vehicle and the other for steering; and a differential receiving the output of both HSTs, whether the system is of the dependent steering type or of the independent steering type.
Otherwise, it is well-known that the vehicle installed with the above mentioned system is provided with a caster or casters in addition to the above mentioned driving axles for improvement of its capacity of steering, that is, so as to enable the vehicle to turn left or right easily and swiftly.
Conventionally, the caster attached to the vehicle has been able to swivel in all horizontal directions in relation to the vehicle body independently of operation of the steering operating means. The steering operating means merely applies difference in rotary speed between left and right driving axles. The caster is laterally swivelled by its following-up the differential rotation of the both axles and is rotated in the direction where it is oriented after the swivelling.
The axis of the caster's own rotation is longitudinally offset from the axis of its lateral swivelling in relation to the vehicle. The caster rotates around its own rotational axis for travelling in such a situation that the swivelling axis and the rotational axis are disposed front and rear. In this regard, the longitudinal direction of the caster is oriented to the travelling direction of the vehicle after the caster has been laterally swivelled. Since the caster is independent of the steering operating means, the lateral swivelling of caster is not performed while the vehicle is stationary but must be performed while the vehicle travels.
Such a construction of caster causes the problem that, while the travelling direction of the vehicle is reversed between forward and backward, the portion of the vehicle to which the caster is attached meanders laterally because the caster is swivelled approximately to an angle of 180° in relation to the vehicle body so as to be longitudinally reversed. Furthermore, the vehicle which has stopped while turning leftward stays in the situation that its caster is oriented leftward to some degree. If the vehicle which has stayed in such a situation starts travelling while turning rightward, the vehicle also meanders laterally because the caster oriented for leftward cornering is laterally swivelled to the direction for rightward cornering for a little while.
Such meandering of the vehicle confuses an operator. Also, while laterally swivelling, the caster is oriented perpendicularly to the travelling direction of the vehicle in a moment so that the resistance of the ground surface against rotation of the caster is maximized, thereby making the volume of the steering HST insufficient to effect its output. Also, in this moment, the caster is dragged without rotation thereby being damaged by the ground surface.
Moreover, since the caster, which is conventionally disposed under the vehicle body, even if it is attached as a front wheel, is invisible to an operator sitting on the seat, the operator feels uneasy especially in such a case supposing the above mentioned meandering of the vehicle.